Method for producing highly corrosion-resistant colored article made of steel

ABSTRACT

The present invention provides a method for producing a highly corrosion-resistant colored article made of steel, which are highly corrosion-resistant and has a variety of vivid surface colors, by subjecting the surface of a nitrided steel article to oxidizing heat treatment to form a colored oxide film layer. The method for producing a highly corrosion-resistant colored article made of steel includes the steps of (a) subjecting an article made of steel to nitriding and (b) subjecting the steel article thus treated in step (a) to surface processing such as abrasion, buffing, polishing or the like, and then to oxidizing heat treatment in an oxidizing atmosphere at 100 to 700° C. for 30 seconds to 100 hours, to form a colored oxide film layer on the surface of the steel article.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing a highly corrosion-resistant colored article made of steel, and in particular, to a method for producing a highly corrosion-resistant colored article made of steel, which are highly corrosion-resistant and has a variety of vivid colors with excellent decorativeness.

2. Description of the Related Art

In general, steel has been used limitedly in industrial materials which are appropriate for the chemical and physical properties inherent to steel. However, along with the development of industries and enrichment in lifestyle, there is an increasing demand for new materials which still have the characteristics inherent to metals and additionally have excellent functions and high added values, as well as colored materials displaying a variety of color tones.

In this regard, development of various colors has been conventionally performed by applying a color paint directly on the surface of a steel article, by coating the surface of a steel article with a color coating composition followed by heat-treating the article, or by immersing a steel article in a metal plating solution to develop the color inherent to the metal or metal compound dissolved in the plating solution.

However, these conventional methods for producing colored steel articles have problems such as peeling off of the paint coated on the steel article over time, environmental pollution due to the use of plating solutions, limitation in developing a variety of colors, and inferior corrosion resistance.

SUMMARY OF THE INVENTION

The present invention has been designed to solve such problems of prior art, and thus an object of the present invention is to provide a method for producing a highly corrosion-resistant colored article made of steel, which has high corrosion resistance and a variety of vivid colors with excellent decorativeness, without polluting the environment.

Other objects and advantages of the present invention will be described in the following, and will be further revealed by Examples of the present invention.

In order to achieve the above-described objects, the present invention provides a method for producing a highly corrosion-resistant colored article made of steel, the method comprising the steps of: (a) subjecting an article made of steel to nitriding and then to abrasion; and (b) subjecting the steel article thus treated in step (a) to oxidizing heat treatment in an oxidizing atmosphere at 100 to 700° C. for 30 seconds to 100 hours, to form a colored oxide film layer on the surface of the steel article.

Here, the oxidizing gas used in the step (b) may be at least one selected from oxygen, air, carbon dioxide and steam, which is used individually or as a mixture of two or more species.

The color obtained in the step (b) is varied according to the temperature and time of the oxidizing heat treatment. At the same temperature for oxidizing heat treatment, as the duration of oxidation is increased, colored oxide film layers are obtained in gold, purple, blue and black colors in this sequence. When the temperature for oxidizing heat treatment is increased, blue or black color is obtained, rather than gold or purple color, even after a short time.

For example, a gold-colored oxide film layer can be obtained after an oxidation time of 18 hours or less at an oxidation temperature of 180° C., or after an oxidation time of 4 hours or less at an oxidation temperature of 220° C. A purple-colored oxide film layer can be obtained after an oxidation time of 12 to 13 hours at an oxidation temperature of 200° C., or after an oxidation time of 6 to 9 hours at an oxidation temperature of 220° C.

Furthermore, at an oxidation temperature of 350° C., black-colored oxide film layers can be obtained even after an oxidation time of 1 hour.

Meanwhile, the process of abrasion in the step (a) may be performed by any one selected from buffing, lapping, abrading and polishing.

In addition, it is preferable that the steel article which has been nitrided in the step (a) has a surface compound layer formed of any one selected from an ε phase, a mixed phase of ε+γ′, and a γ′ phase.

According to the method for producing a colored steel article of the present invention as described above, since a colored oxide film layer is formed by subjecting the nitrided steel article to oxidizing heat treatment using an oxidizing gas, a variety of colors which cannot be developed by conventional plating methods or coating methods can be developed. Also, the steel article thus treated has excellent decorativeness, thus being applicable to a wide range of applications, and has improved corrosion resistance. Therefore, a highly corrosion-resistant colored steel article with excellent decorativeness can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a flow diagram showing a method for producing a highly corrosion-resistant colored article made of steel according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described in more detail with reference to the attached drawing.

The FIGURE is a flow diagram showing the method for producing a highly corrosion-resistant colored article made of steel according to the present invention.

Referring to the FIGURE, the surface of an article made of steel is subjected to nitriding heat treatment to form a surface compound layer composed of an ε phase, a mixed phase of ε+γ′, or a γ′ phase (S10), according to the present invention.

Here, the thickness of the surface compound layer is preferably from 2 μm to 100 μm.

The temperature of the nitriding heat treatment is preferably from 300 to 700° C., while the duration of the nitriding heat treatment is preferably from 1 hour to 20 hours.

Here, the nitriding heat treatment can be performed by any nitriding heat treatment process, such as gas nitriding, plasma nitriding, salt bath nitriding, vacuum nitriding or the like. A representative method for such nitriding heat treatment may be exemplified by the method for producing steel articles in substitution of the plating treatment suggested by the present Applicant in Korean Application No. 1993-10873.

The steel material used in the article to be treated is not particularly limited, and may be exemplified by cold-rolled steel, carbon steel, alloy steel or the like.

The steel article thus nitrided is then subjected to cooling, and the process of cooling can be performed by air cooling, furnace cooling, or water cooling (S20).

The steel article that has undergone the process of cooling is then subjected to a process of abrasion such as buffing, lapping or polishing, in order to obtain uniform roughness (S30).

Subsequently, the steel article thus obtained after the process of abrasion, and an oxidizing gas are introduced into a furnace (S40) to perform the process of forming a colored oxide film layer.

The oxidizing gas may be any one selected from oxygen, air, carbon dioxide and steam, which are used individually or as a mixture of two or more species, to form a colored oxide film layer on the surface of the steel article, and the oxidizing heat treatment can also be performed using nitrogen gas together with the oxidizing gas.

The process of forming a colored oxide film layer is performed by subjecting the surface of the nitrided steel article to oxidizing heat treatment to form a colored oxide film layer (S50), and the oxidizing heat treatment is performed under the conditions such as a temperature ranging from 100° C. to 700° C. for 30 seconds to 100 hours, in accordance with the color intended to be manifested.

If the temperature of oxidizing heat treatment is below 100° C., the oxidizing heat treatment is not properly performed, and an iron oxide layer cannot be obtained as desired. If the temperature of oxidizing heat treatment is above 700° C., phase change and decomposition of the compound at the surface layer occur.

Furthermore, if the time for oxidizing heat treatment is shorter than 30 seconds (less than 3 seconds in the case of high frequency treatment), in case of using the aforementioned furnaces, the temperature for forming a colored oxide film layer, at which oxidization treatment is performed, may not be reached, and thus it is unlikely that a colored oxide film layer is obtained as desired. If the time exceeds 100 hours, the color does not change anymore from black color, and the extra time taken is unnecessary in view of economic efficiency.

Specifically, the surface color of the colored oxide film layer formed after the oxidizing heat treatment may be manifested as gold, purple, blue or black according to the temperature, time and atmosphere of the oxidizing heat treatment. The composition of the colored oxide film layer mainly comprises FeO, Fe₂O₃ or Fe₃O₄, as a single phase or a mixed phase.

It is preferable that the thickness of the colored oxide film layer is 0.05 μm to 5 μm, because the colors viewed by the eyes are under the influence of scattering or interference of light in accordance with the thickness of the surface compound layer and the surface morphology of the oxide layer.

Thereafter, the steel article on which a colored oxide film layer has been formed through the process of oxidizing heat treatment is cooled by any one method selected from air cooling, surface cooling, water cooling and oil cooling (S60).

Therefore, while conventional methods for coloring articles made of steel cannot have desired colors manifested because these methods involve providing plating layers on the surface of the steel articles, thus preventing Fe from being brought into contact with O₂; the method for producing a colored steel article of the present invention allows color manifestation through oxidation of the steel material, and develops a variety of colors based on gold, purple, blue and black colors according to the oxidizing heat treatment conditions, thus imparting vivid and varied colors and excellent corrosion resistance to steel articles. Accordingly, the steel articles thus produced by the method of the present invention can be applied to a wide range of applications including interior decorations, various structures and construction materials.

Meanwhile, the heat treatments that are used for the nitriding heat treatment and the oxidizing heat treatment can be performed using a pit type furnace equipped with stirring fans at the top and the bottom and having gas introduced at the top or the bottom, a sealed quench furnace, a fluidized bed furnace, a meshbelt type continuous furnace consisting of three or more chambers respectively equipped with stirring fans at the top and the bottom, a salt bath furnace or a plasma furnace. However, other furnaces can also be used as long as they can satisfy the oxidizing heat treatment conditions.

Moreover, it is also possible according to the present invention, to produce a colored steel article by applying a high frequency induction heating method, in addition to the oxidizing heat treatment method. When the high frequency induction heating method is applied, a desired color can be developed within a short time of about 3 seconds to 1 hour using the aforementioned oxidizing gas at the aforementioned treatment temperature.

As discussed above, the method for producing a colored steel article of the present invention can be used to develop a variety of highly decorative colors which cannot be developed by conventional plating methods and coating methods, and thus can produce highly corrosion-resistant colored steel articles with excellent decorativeness, which have further improved corrosion resistance through oxidizing heat treatment, and which are applicable to a wide range of applications.

EXAMPLE 1

An article made of carbon steel and an article made of an alloy steel were subjected to nitriding in an atmosphere of a gas mixture containing 50% by volume of NH₃, 5% by volume of CO₂ and 45% by volume of N₂ at 560° C. for 3 hours, and then to air cooling. The thickness of the resulting surface compound layer was 17 μm, and the obtained phase was ε-phase.

Subsequently, the nitrided steel articles were buffed and placed in a furnace, in which the nitrided steel articles were subjected to oxidizing heat treatment in an air atmosphere at 180° C. for 9 hours and then to air cooling. As a result, a colored oxide film layer having a gold surface color was formed.

EXAMPLE 2

First, an article made of carbon steel and an article made of an alloy steel were subjected to nitriding by salt bathing at 570° C. for 3 hours and to water cooling, thus to produce steel articles having surface compound layers of a mixed phase of ε+γ′ formed thereon. The thickness of the resulting surface compound layers was 20 μm.

Subsequently, the steel articles thus nitrided were polished. The thickness of the polished surface compound layers was 18 μm, and the surface roughness was 1.0 μm Ra.

Then, the steel articles were placed in a furnace and were subjected to oxidizing heat treatment in an atmosphere containing air and steam at a volume ratio of 50:50 at a temperature of 200° C. for 2 hours, and then to air cooling. Thus, colored steel articles having colored oxide film layers with a gold surface color formed thereon were produced.

EXAMPLE 3

An article made of carbon steel and an article made of an alloy steel were subjected to nitriding in an atmosphere of a gas mixture containing 50% by volume of NH₃ and 50% by volume of RX at 580° C. for 3 hours, and then to water cooling. The resulting surface compound layers were 22 μm in thickness, and comprised a mixed phase of ε+γ′.

The steel articles were abraded, and then were subjected to oxidizing heat treatment in an atmosphere of a gas mixture containing air and carbon dioxide at a volume ratio of 50:50 at 220° C. for 4 hours, and to water cooling. As a result, the steel articles attained a gold surface color.

EXAMPLE 4

An article made of carbon steel and an article made of an alloy steel were subjected to nitriding in an atmosphere of a gas mixture containing 50% by volume of NH₃ and 50% by volume of RX at 580° C. for 3 hours, and then to water cooling. The resulting surface compound layers were 22 μm in thickness, and comprised a mixed phase of ε+γ′.

The steel articles were abraded, and then were subjected to oxidizing heat treatment in an air atmosphere at 260° C. for 1 hour, and to water cooling. As a result, the steel articles attained a purple surface color.

EXAMPLE 5

An article made of carbon steel and an article made of an alloy steel were subjected to plasma nitriding in an atmosphere of a gas mixture containing nitrogen, hydrogen and methane at 550° C. for 5 hours under the conditions of a total pressure of 5 torr, a voltage of 500 V, and a pulse of 8 kHz. The resulting surface compound layers were 25 μm in thickness, and were composed of a γ′ phase.

The steel articles were subjected to oxidizing heat treatment in an atmosphere of a gas mixture containing nitrogen and oxygen at a volume ratio of 30:70 at 260° C. for 5 hours, and to oil cooling. As a result, the steel articles attained a blue surface color.

EXAMPLE 6

An article made of carbon steel and an article made of an alloy steel were subjected to plasma nitriding in an atmosphere of a gas mixture containing nitrogen, hydrogen and methane at 550° C. for 3 hours under the conditions of a total pressure of 5 torr, a voltage of 500 V, and a pulse of 8 kHz. The resulting surface compound layers were 15 μm in thickness, and were composed of a γ′ phase.

The steel articles were subjected to oxidizing heat treatment in an atmosphere of a gas mixture containing nitrogen and oxygen at a volume ratio of 30:70 at 350° C. for 1 hour, and to oil cooling. As a result, the steel articles attained a black surface color.

Corrosion Resistance Test for Steel Articles

Each of the colored steel articles that have undergone the process of forming colored oxide film layers was subjected to brine spraying for 72 hours, but no rusting was observed.

The method for producing a highly corrosion-resistant colored article made of steel of the present invention as described above provides the following effects.

First, the method is a pollution-free method which is distinguished from conventional plating methods and coating methods, thus preventing environmental hazards.

Secondly, since colored articles made of steel having a variety of vivid colors with high decorativeness, which cannot be developed by conventional methods can be produced, the colored articles are applicable to a wide range of applications.

Thirdly, the colored steel articles thus produced through nitriding and oxidizing heat treatments have excellent corrosion resistance.

As discussed above, although the present invention has been described by a limited number of examples and drawing, the present invention is not intended to be limited thereby, and those having ordinary skill in the art to which the present invention pertains will understand that various alterations and modifications are also possible within the technical idea of the present invention and a scope equivalent to the following claims. 

1. A method for producing a highly corrosion-resistant colored article made of steel, the method comprising the steps of: (a) subjecting an article made of steel to nitriding and then to abrasion; and (b) subjecting the steel article thus treated in step (a) to oxidizing heat treatment in an oxidizing atmosphere at 100 to 700° C. for 30 seconds to 100 hours, to form a colored oxide film layer on the surface of the steel article.
 2. The method according to claim 1, wherein the oxidizing atmosphere comprises oxygen, air, carbon dioxide or steam, either individually or as a mixture of two or more species.
 3. The method according to claim 1, wherein the process of abrasion is performed by any one selected from buffing, lapping, abrading and polishing.
 4. The method according to claim 1, wherein the heat treatment used for the nitriding or oxidizing heat treatment is performed using any one selected from a pit type furnace equipped with stirring fans at the top and the bottom and having gas introduced at the top or the bottom, a sealed quench furnace, a fluidized bed furnace, a meshbelt type continuous furnace consisting of three or more chambers respectively equipped with stirring fans at the top and the bottom, a salt bath furnace, a plasma furnace, and a high frequency induction furnace. 